This invention relates to calcium-aluminum system hydrogen absorbing alloys.
Hydrogen absorbing alloys are characterized by reacting directly with hydrogen and thus quickly absorbing a large amount of hydrogen, while desorbing the hydrogen thus absorbed. Accordingly, they enable reversible absorption and desorption of hydrogen. Thus there have been actively made developments regarding techniques in the use of these hydrogen absorbing alloys mainly in the field of energetic technology. Hydrogen absorbing alloys are being applied to various purposes including the storage and transport of hydrogen, energy conversion media and negative electrodes for some secondary batteries.
In order to put a hydrogen absorbing alloy into practical use, it is generally necessary that the hydrogen absorbing alloy satisfy the following requirements.
(1) Having such a hydrogen absorption pressure and a hydrogen dissociation pressure as to facilitate handling within the operating temperature range. PA1 (2) Showing high rates of hydrogen sorption within the operating temperature range. PA1 (3) Having a large chargeable hydrogen capacity within the operating temperature range and under such a pressure as to facilitate handling. PA1 (4) Being easily activated during initial hydriding. PA1 (5) Showing a small difference between hydrogen pressures required for hydrogen absorption and hydrogen desorption (i.e., hysteresis). PA1 (6) Being highly durable when subjected to repeated absorption and desorption over a long period of time. PA1 (7) The cost of materials being low. PA1 (8) The alloy per se being not heavy.
Regarding these points, there have been publicly known Laves phase hydrogen absorbing alloys with the C15-type structure such as group IVa metal-3d transition metal system alloys (Ti-Cr system, Zr-V system, Zr-Mn system, etc.) and rare earth metal-3d transition metal system alloys [La-Ni system, Mm (misch metal)-Ni system, etc.].
On the other hand, publicly known hydrogen absorbing alloys containing Ca as the main component are exemplified by Ca-Ni system and Mg-Ca system alloys. Although the Ca-Ni system alloy has a large hydrogen storage capacity and can easily undergo the initial activation, it requires a large amount of Ni which is an expensive and heavy element. Although the Mg-Ca system alloy is a light one, it has not been put into practical use because it suffers from some problems, i.e., (a) requiring prolonged initial activation at a high temperature; (b) having a low equilibrium hydrogen dissociation pressure at ordinary temperatures; and (c) being poor in oxidation resistance.
Therefore attempts have been made to impart the characteristics (1) to (8) as described above to a hydrogen absorbing alloy or to improve these characteristics to thereby give a hydrogen absorbing alloy having an improved practical availability by, for example, making the composition ratio of a hydrogen absorbing alloy (mainly the above-mentioned Laves phase hydrogen absorbing alloy or one containing Ca as the main component) nonstoichiometric or making multi-components. In the case of Ca-Ni system or Mg-Ca system alloys, for example, attempts have been made to replace some part of the alkaline-earth metal with a rare earth element or to replace some part of Ni with other transition elements, etc.
When used in products such as negative electrodes of nickel-metal hydride (Ni-MH) secondary batteries or fuel cells, however, these alloys are undesirable from the viewpoints of lightness and cheapness. Thus they leave much room for improvement regarding these points.